1. Field of the Invention
This invention relates to a computer system using a memory which is required to be refreshed in a certain interval cycle.
This invention also relates to a method for refreshing a memory of a computer system in which the memory is required to be refreshed in a certain interval cycle.
More particularly, this invention relates to a computer system using a dynamic random access memory and a method for refreshing it while the system enters a standby operation mode.
Furthermore, and more particularly, the invention relates to a micro-computer system using a DRAM memory.
Furthermore, and more particularly this invention relates to a microcomputer system for a refreshing a DRAM by switching an interval or auto refresh mode and a self refresh mode.
2. Discussion of the Background
A computer system uses an outside memory device for storing data such as operational system programs. In particular, a micro-computer system, uses two kinds of memory devices. One is a static random access memory (hereinafter refers to as "SRAM") and the other is a dynamic random access memory (hereinafter referred to as "DRAM").
The SRAM can maintain its memory contents even when a clock generator in the system stops providing timing clock signals during a standby mode of system operation, as far as electric power is supplied to the system.
Because the ciock generator is one of the most electric power consuming devices in the system, it is beneficial, particularly for a small size computer, like a micro-computer system, to stop providing timinS clock signals during a standby mode of system operation.
Unfortunately, the SRAM is too expensive to use as a memory for a micro-computer system. So, the DRAM is usually used for a memory in a micro-computer system. However, to the SRAM, the DRAM is required to be refreshed in a certain interval cycle for storing memory contents.
Consequently, a computer system using a DRAM can not stop the clock generator because an interval memory refresh operation must be maintained.
This means that a system using a DRAM can never enter into a standby operation mode in order to save electric power consumption, even though this is required as a basic function of a small size computer system.
Furthermore, in a case of an instantaneous disconnection of an electric power source or a service interruption of an electric power supply, the same problem for destroying memory contents in a DRAM will appear because the memory refresh control circuit stops when a clock generator stops providing clock timing signals.
In order to prevent such problems, it is proposed to have a special clock generator for use only for memory refresh operation. However, this make the chip size larger, making mounting of the chip in a small size computer difficult.
Furthermore, this solution makes it basically impossible to achieve a low electric power consumption system since such a special clock generator also requires an electric power supply in order to provide clock signals for memory refresh operation even when a system enters to a standby mode.
Increasing noises also appears if a micro-computer system uses the plural clock generators. Because a clock generator oscillates a high frequency signal, it causes to become a noise source in a system. Accordingly, to provide plural clock generators increases such noise sources and it causes many bad influences for normal system operation.